1. Field of the Invention
This invention relates generally to a protective circuit for a transistor amplifier, and more particularly to a protective circuit for a transistor amplifier which operates without misoperation in the case of using a reactive load and operates in response to a load impedance and a surge voltage of a power supply voltage.
2. Description of the Prior Art
In the prior art, many protective circuit arrangements for a transistor amplifier have been disclosed which have a comparison circuit for comparing a load voltage and a load current, but they have an essential disadvantage in that they mis-operate in the case where the load is a reactive load. The characteristics of a typical prior art circuit will be described with reference to FIG. 1.
The limiting (protective) line of a prior art protective circuit can be generally expressed by, for example, a line a for the output current I and output voltage V of the amplifier as shown in FIG. 1. In the case where the load is a pure resistor or nearly pure resistors for the above protective characteristics, the load line of such a protective circuit can be expressed by a line b in FIG. 1 and hence the protective circuit performs a normal protective operation. In the case, however, where the load is, for example, a condenser-speaker which is rich in reactance components, the load line is expressed by an ellipse c shown in FIG. 1 and the elliptic curve c intersects the limiting line a. Thus, the protective circuit operates. However, in the latter case, even if the load line becomes the ellipse c, the ellipse c exceeds the limiting line a for a short time period.